Here's another idea that just occurred to me. We could say that
shadows, outlines, etc. can never cause a scrolling mechanism to
appear, but just leave it at that.
In other words you never let the shadows cause scrollbars to be
created (or destroyed), but if scrollbars happen to already be there
(because of some other overflow), then you can safely include the
visual overflow as part of the scrolling area.
dave
(hyatt@apple.com)